Hi Martin,
If you are developing on Windows, you can use the EyeX SDK for .NET on both the Tobii REX and Tobii EyeX Controller.
The most significant differences are:
– The EyeX Controller costs less
– The EyeX Controller requires USB-3 (SuperSpeed)
– The EyeX Controller only runs on Windows (at the moment)
For more info, check out this thread:
http://developer.tobii.com/community/forums/topic/differences-between-tobii-rex-and-tobii-eyex-dev-kit/